Disposition of cyproheptadine in rats, mice, and humans and identification of a stable epoxide metabolite.

Radioactivity was excreted in the urine and feces of rats, mice, and humans after a dose of 14C-cyproheptadine. The major metabolite in rat urine was unconjugated, but the majority of radioactive materials in mouse and human urine were conjugated with glucuronic acid. Identification of the rat urinary metabolite of cyproheptadine as an epoxide was accomplished with mass spectrometry and other methods. The rat metabolite was 10.11 -epoxydesmethylcyproheptadine and accounted for about 25% of a 45-mg dose of cyproheptadine per kg. Only a small amount of this epoxide was found in mouse urine, and none was apparent in the urine of two humans who received 5 mg of the drug. Dihydrodiols, which could arise by epoxide hydrase hydrolysis of possible 10.11-epoxy metabolites, were not found in the urine of any of the species studied. The spoxide found in rat urine appears to be unusually stable to in vivo hydrolysis. Possible implications of these results in the species-selective pancreotoxicity of cyproheptadine in the rat are presented.